An evaluation of canopy gaps in restoring wildlife habitat in second growth forests of Southeastern Alaska1 Paul Alaback FINAL REPORT February 20, 2010 1 A cooperative project with The Nature Conservancy-Alaska, Thorne Bay Ranger District and Craig Ranger District, Tongass National Forest, and POWTEC. Executive Summary We report on our initial findings from a two year study on a 20-year remeasurement of canopy gap treatments in second growth forests on Prince of Wales Island in Southeastern Alaska. Seventy-six gaps were selected for sampling representing a broad geographic, and ecological range of stand conditions throughout the region. Our analysis of these plots suggest that canopy gaps represent one of the most effective techniques for long-term improvement of habitat for deer and associated wildlife species in second growth forests on Prince of Wales. Our data shows statistically significant increases in species diversity, understory cover, forb biomass, and shrub annual growth for gap plots as compared to either thinned or unthinned controls. Canopy gap treatments create habitats that are on average 4 times the deer carrying capacity of our thinned second growth stands in the summer or over 8 times the carrying capacity of thinned sites in the winter. Within a gap there is as much summertime blueberry (Vaccinium) biomass as in typical old growth forests. A simple model (TONGASS GAP) was constructed for estimating the overall effect of canopy gaps on deer habitat at the stand level and also to provide managers with a tool to examinine tradeoffs between gap size, density and deer habitat for closed-canopy second growth stands. From this model we estimate as much as a four-fold increase in deer carrying capacity for winter habitats when up to 50% of a given stand has gap habitat. More typically gap treatments have created 5-10% gap habitat in clearcut units which should result in as much as a 50% increase in winter deer carrying capacity. It appears likely that these gaps will persist well into the future of these stands, since there was no significant increase in tree saplings following gap treatment, and there was no detectable influence of gap size on vegetation response. It will be highly desirable to continue monitoring canopy gap treatments to determine the overall longevities of these gaps, to determine the functional upper limit of gap size, and to determine the best ways to incorporate these treatments into overall stand and landscape management in the region. Scientific and Management Context One of the most difficult and long-standing conservation issues facing residents and land managers in Southeast Alaska has been the log-term impact of clearcut logging old-growth forests on wildlife habitat. Studies dating back to the 1960’s and 1970’s documented how logging has the potential to negatively effect many species of interest such as Sitka black- tailed deer, wolves, and goshawks in this region (TLMP planning documents, Wallmo and Schoen 1979, Alaback 1982, Hanley et al. 1985). Over 400,000 acres of highly productive, old-growth temperate rainforest in southeast Alaska have been logged since 1950. This timber harvest has been concentrated in the most productive and economically valuable forest stands at low elevations which have historically been important areas for people in local communities for hunting and subsistence, and as habitats for critical 2 wildlife species. Over half of the timber harvest has taken place on one island – Prince of Wales, in southern Southeast Alaska, since it has the largest concentration of easily accessible productive forest habitats in the region. In Southeast Alaska there are many specific ecological factors which explain why logging can have such a negative impact on key wildlife species in this region. Most logging has occurred in low-elevation valley bottoms (<1000’) which provide critical habitat for wildlife, especially during times of heavy snow cover. Removal of old-growth forest and its replacement by second-growth forest affects winter habitat for deer in two specific ways: loss of snow shedding capability of complex old-growth canopies (effects mobility and foraging efficiency of deer) and loss of a productive understory plant community (provides forage quality and quantity). Although clearcut harvesting does produce an immediate flush of high quality understory biomass, it typically lasts only 10-25 years, and is not available to deer during the periods of heavy snow. The greatest impact occurs three or more decades after logging, during the “stem exclusion” phase of forest stand development, when the densely stocked and rapidly growing young conifers shade out most of the important plant species for deer and other wildlife species. The stem exclusion phase lasts for as much as 150– 200 years so can create a long-lasting deficit of wildlife habitat for a given watershed or region, unless an effective restoration strategy can be developed (Alaback 1982). Over the succeeding 30 years since this issue of logging impacts has been understood scientifically, most efforts at restoration have centered on using techniques such as silvicultural thinning to stimulate understory vegetation growth (Hanley 2005, McClellan 2005). While thinning can be effective in improving wildlife habitat 5-10 years following treatment, one of the key limitations of this treatment is its relatively short longevity. This should not be too surprising since thinning is an agricultural technique dating back to at least the 1700’s designed to stimulate the growth of crop plants. By thinning a forest, the nutrients and other resources of a given forest are concentrated on a smaller number of trees resulting in increased growth rates and individual tree productivity as predicted by the -3/2 law (e.g. Oliver and Larson 1990). As a by-product, thinning can stimulate understory vegetation at first, since sunlight and nutrients become more available immediately after treatment. Soon, generally not more than 15 years, crop trees expand their branches and create a dense overstory canopy which shades out understory forage plants once again. While more intense thinning treatments (wider average tree spacing) may lengthen this process to a certain extent, data available to date suggests that on productive sites thinnings even up to a spacing of 20’ will still produce only short-term benefits to wildlife habitat (Alaback, unpublished data). One relatively unstudied experimental treatment that appears to hold great merit for improving understory vegetation forage availability and diversity is the creation of artificial canopy gaps. Gaps represent small (<1/2 acre, or less than 160’ in diameter) clearings that simulate wind disturbance or small patch tree mortality characteristic of old 3 growth forests in Southeast Alaska (Juday and Ott 2002). Each gap is large enough to provide enough canopy opening and sunlight to produce significant forage, yet appears to be small enough to prevent a “conifer flush” typical of larger clearcuts or strip thinning prescriptions. Thus, the benefits of understory productivity are expected to last much longer than with conventional thinning. Moreover, understory flora within the gap includes evergreen forbs that represent a critical food source for a number of species in winter, as well as providing for snow interception and thermal cover along the edges. The forest structure created by canopy gaps would be expected to be more similar to the patchy forest conditions that characterize old-growth forests than what would result from any of the thinning treatments that have been studied. Gap thinning applied at a landscape scale, combined with conventional thinning prescriptions, represents an innovation that may meet multiple objectives for conservation of biodiversity and timber management on clearcut landscapes. Only a fraction of the area generally requires a gap treatment (<5-10%), so the additional cost is small compared with the benefit of increased understory productivity and diversity within an otherwise unproductive landscape. Meanwhile, at the same time conventional thinning prescriptions will serve to meet timber management objectives within the larger landscape matrix. Within the managed landscape, this new regime moves beyond the artificial dichotomy of conservation versus development, and toward a more integrated approach where larger landscapes serve to meet multiple biodiversity and resource management objectives. Nearly 600 gaps were installed on Prince of Wales Island from 1983-1993, and over the past few years many more have been installed across the region. The stated objectives are to “maintain forage production and habitat diversity for deer in stands 25 to 35 years old” and to “simulate old growth habitat conditions by opening up holes in the overstory to stimulate production of understory forbs and shrubs while also providing snow intercept”. While opinions of managers and residents vary as to the perceived value of these treatments, there has been no detailed scientific study of the effectiveness of canopy gap treatments in Alaska until we initiated this study in 2008. The Forest Service made two major efforts to monitor a sub-set of those gaps in 1990 and again in 1994 (Demeo 1990, Knotts and Brown 1995). Because of the efforts of the Forest Service to monitor and document these treatments, and the efforts of several forward-thinking managers to carefully archive records and data relating to these treatments we had the unique opportunity to examine the 20 year response of vegetation to canopy gap treatments in this study. 4 Objectives of this study: General: To determine the overall effectiveness of canopy gaps in restoring productivity of understory vegetation and wildlife habitat to second growth stands on Prince of Wales Island. Specifically: 1. Document long- term response of understory vegetation to canopy gap treatments on Prince of Wales Island. 2. Determine the influence of canopy gap size on: a)vegetation composition, b)forage quantity, and c)tree regeneration. 3. Determine long-term effects of thinning on vegetation response to canopy gaps.